Resonant Photoemission Measurements Research Articles

Valence-band and resonance-photoemission study ofLa2CuO4+xsingle crystals

Angle-resolved resonance-photoemission measurements at 15{endash}30 and 100{endash}130 eV photon energy ranges were performed from different principal planes of La{sub 2}CuO{sub 4+x} single crystals. Resonance photoemission near the La-4d threshold shows the presence of states in the valence band associated with La within the energy region 2{endash}5.5 eV. At low photon energies near the La-5p threshold (16.5 eV) with the normal emission aligned in the (001) direction (c{perpendicular} surface), valence-band spectra display nonmonotonic photon energy dependence of a 6-eV binding-energy feature, i.e., beyond the main La density of states in the valence band. Along with the absence of such behavior for normal emission from the other principal planes (100) and (010) (c{parallel} surface geometry), this fact implies that surface termination effects need to be taken into account to explain these results. The binding energy of the resonant feature and the resonance photon energy are slightly different for photon polarization vector parallel and perpendicular to the Cu-O planes of the crystal. Measurements were also done on another La-based compound, La{sub 0.7}Sr{sub 0.3}MnO{sub 3}, having similar La-O plane but much smaller c-axis parameter. This manganese oxide demonstrates the same nonmonotonic dependence of the 6-eV binding-energy feature, but in a wider photon energy region.more » On the basis of this comparison, it is tentatively suggested that the intensity modulation is governed not only by the core-level threshold, but may carry information of bulk Brillouin zones as well. {copyright} {ital 1997} {ital The American Physical Society}« less

2p resonant photoemission study ofTiO2s

Resonant photoemission measurements of ${\mathrm{TiO}}_{2}$ (100) at the Ti ${\mathrm{L}}_{2,3}$ edge are reported. For transitions from 2p to 3d states of ${\mathrm{e}}_{\mathrm{g}}$ symmetry there is a resonance enhancement of the intensity of the band-gap defect states by a factor of 6. The valence band shows resonance enhancements for transitions to states of both ${\mathrm{e}}_{\mathrm{g}}$ and ${\mathrm{t}}_{2\mathrm{g}}$ symmetry. The maximum enhancement of 3 for the high binding energy indicates that the whole of the O 2p derived valence band is hybridized with Ti 3d states, with the higher binding-energy part being more strongly mixed. The defect-state resonances are broader than the absorption resonances and have a width of about 4 eV due to the multiplet structure.

Zn, Mn and Te K-edge EXAFS studies of the diluted magnetic semiconductor

Extended x-ray absorption fine-structure measurements at the Zn, Mn and Te K edges of the diluted magnetic semiconductors have been performed to investigate the local coordination around each of the atoms. The nearest-neighbour and next-nearest-neighbour interatomic distances are found to be almost unchanged on changing the Mn concentration (x). These results indicate that the tetrahedral and clusters are embedded in the alloys with a well-preserved form, in agreement with the x-independent Mn 3d partial density of states derived from resonant photoemission measurements.

The experimental density of states of Pd 4d and Ti 3d in the intermetallic compound

Photoemission measurements have been made of the intermetallic compound utilizing synchrotron radiation in the energy range 40 - 180 eV. The valence band extends to a binding energy of 7 eV. Measurements made at the Cooper minimum for Pd 4d indicate that the highest density of states for Ti 3d is at the Fermi edge and that it decreases towards higher energy. Using this density of states and correcting for the photoemission cross-sections with energy, it has been possible to determine the density of states for Pd 4d which also contributes to the Fermi level. Resonance photoemission measurements are consistent with this conclusion and also indicate that the Pd 4d band is not completely full.

Photoemission study of MnZnTe(110) interface

We have performed Mn 3p-3d resonant photoemission measurements on MnZnTe(110) interfaces. The spectra for the Mn coverages below 1 Å are similar to those of bulk diluted magnetic semiconductor Zn 1− x Mn x Te alloys. The result suggests that the Mn deposition at room temperature onto ZnTe(110) surface leads the formation of surface Zn 1− x Mn x Te alloys. The Mn 3d partial densities of states in the surface alloys are almost independent of the Mn concentration in agreement with the bulk alloys.

Angle-resolved and resonant photoemission study of Sr 2RuO 4

Resonant and angle-resolved photoemission measurements were performed for the non-cuprate layered superconductor Sr 2RuO 4 ( T c = 0.93 K). Resonant valence band photoemission spectroscopy shows the existence of a correlation induced satellite to the main d band across the Ru 4 p-4 d threshold. Angle-resolved measurements show a surprisingly narrow band of about 50 meV width and positioned at 26 meV below the Fermi level, along the ΓX and ΓZ directions. This band, arising fom Ru 4 d-O 2 p antibonding states, is nearly dispersionless over a wide range of k points about the zone boundary along the Γ- Z direction (Ru-O direction), reminiscent of the extended Van-Hove singularity observed in the cuprates. The results emphasize the presence of strong correlations, substantial charge-transfer character to states at the Fermi level and the existence of a singular flat band near the Fermi-level in this low T c layered superconductor, just like the cuprates.

Resonant photoemission study of the electronic structure of K 0.3MoO 3

We report resonant photoemission measurements performed on both stoichiometric and defective (20 1 ) surfaces of the quasi-one dimensional oxide conductor K 0.3MoO 3. We investigate the resonant enhancement of the photoemission intensity in the O 2 p band and of defect-related emission 2 eV below the Fermi level. The strong Fano-like resonance profile of the defect emission allows us to identify the defects as oxygen vacancies. The destructive nature of resonant photoemission for this system is revealed, as the core hole excitations that are necessary for the resonant photoemission process also lead to photon stimulated desorption.

Substitution-induced mid-gap states in R x Ba1?x TiO3?? whereR = Y, La, and Nd

Changes induced in the electronic structure of BaTiO3 by substitutions of R = Y, La or Nd for Ba to form the mixed oxides RxBa1−xTiO3−δ have been investigated using ultraviolet photoelectron spectroscopy. Substitution of formally R+3 ions for Ba+2 leads to the introduction of filled states in the band gap that are shown by resonant photoemission measurements to have significant Ti 3d character, consistent with a Mott-Hubbard insulator description for these oxides. It is suggested that the dominant factor is electron-electron correlation and this leads to the estimatesU∼Δ∼3 eV for this system, whereU is the correlation energy for the 3d and Δ is the charge transfer energy. Changes are observed in the photoelectron spectral shape for these states as a function of increasing substitution in the Nd system and discussed in the context of the opening of the Hubbard gap.

Transition metal impurities and band offsets in wide gap II–VI semiconductors: Zn1−xMnxSe(Ni) compounds

Zn1−xMnxSe(Ni) solid solutions have been analysed by absorption, reflectivity and resonant photoemission measurements. Valence band offsets for the compounds with different Mn concentrations were estimated from the threshold energy ħωth of the photoionization of Ni2+ ions as well as from photoemission spectra. The contribution of the Mn3d derived states to the valence band was determined.

Electronic structure of hole-doped Sr1+xLa1-xFeO4 studied by UPS and XAS.

The electronic structure of ${\mathrm{K}}_{2}$${\mathrm{NiF}}_{4}$-type ${\mathrm{Sr}}_{1+\mathit{x}}$${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{FeO}}_{4}$ compounds (0\ensuremath{\le}x\ensuremath{\le}0.3) was studied by ultraviolet-photoemission and x-ray-absorption spectroscopies. Resonant photoemission measurements demonstrated that although the satellite feature of ${\mathrm{Sr}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{FeO}}_{4}$ was attributed to ${\mathit{d}}^{4}$ final states, the whole valence band was constructed from well mixed Fe 3d and O 2p states. It was suggested that the parent ${\mathrm{Sr}}_{1+\mathit{x}}$${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{FeO}}_{4}$ compound is intermediate between Mott-Hubbard and charge-transfer insulators. O K-edge and Fe K-edge x-ray-absorption measurements showed that hole bands composed of O 2p and Fe 3d character were formed in the original conduction-band edge upon hole doping, while the valence-band structure obtained by nonresonant photoemission measurements did not change. Changes in the electrical and magnetic properties were correlated with those in electronic structure.

Resonant photoemission determination of the valence electronic structure of Cr 2O 3

Resonant photoemission measurements have been performed on well characterized, stoichiometric (10 1 2) surfaces of single-crystal Cr 2O 3 using synchrotron radiation. A strong resonant enhancement of the highest-lying valence-band states is observed across the Cr 3p → 3d excitation threshold. In light of the similarity of the resonance in Cr 2O 3 with that of Ti 2O 3 and V 2O 3, we conclude that Cr 2O 3 is a Mott-Hubbard insulator. Cr 3d - O 2p hybridization is evidenced by the presence of the similar, but weaker, resonant enhancement in the O 2p valence-band region.

Role of Coulomb repulsion in 4f orbitals in electrical excitation of rare-earth impurities in semiconductors

The role of the on-site Coulomb repulsion energy in the electrical excitation of rare-earth ions by minority carrier injection in semiconductors is examined. Resonant photoemission measurements of the interfacial electronic energy level alignment in NdF3/silicon heterojunctions show that the large Coulomb repulsion energy of the atomiclike 4f orbitals prevents the direct excitation of the 4f levels by either electron or hole capture.

Resonance photoemission from single crystalline Bi 2Sr 2CaCu 2O 8 at the Cu 3p absorption edge

Resonance photoemission measurements of the high- T c superconductor Bi 2Sr 2CaCu 2O 8 at the Cu 3p absorption edge are reported. The resonance behaviour of valence states lying between 10 and 14 eV binding energy is similar to that observed from CuO, confirming their d 8 character. A resonance of the valence band states is also observed, although neither an enhancement nor a resonance is observed in Fermi level states. The latter effect is consistent with these final states having little d 8 character.

Determination of valence states in Fe3O4 by resonant photoemission

We report resonant photoemission measurements across the Fe 3p→3d photoabsorption threshold from the (110) surface of cleaved single-crystal Fe3O4 using synchrotron radiation. The resonant enhancement effect is used to distinguish the Fe 3d-derived states from the overlapping O 2p states in the valence band. The Fe 3d-derived states are found to extend about 18 eV below the Fermi level. Constant-initial-state spectroscopy across the Fe 3p excitation threshold reveals that this large valence bandwidth is due to a significant amount of cation–ligand hybridization; this result is in accord with recent configuration interaction cluster calculations of the photoemission final states in iron oxides reported by Fujimori et al. The electronic levels that are primarily associated with the different Fe2+ and Fe3+ cation environments in Fe3O4 are identified based on a comparison with photoemission spectra measured from cleaved FexO (x≂0.945) and α-Fe2O3 single crystals. Upon O2 adsorption, the Fe3O4(110) surface is rapidly oxidized, as evidenced by a decrease in the photoemission features associated with the Fe2+ valence state.

Electronic structure of MnO studied by angle-resolved and resonant photoemission.

Results are reported from angle-integrated, angle-resolved, and resonant photoemission measurements on cleaved MnO single crystals using synchrotron radiation. Normal-emission spectra from a cleaved (100) surface indicate that the relative dispersion of Mn 3d states in the valence band is less than \ifmmode\pm\else\textpm\fi{}0.1 eV along \ensuremath{\Gamma}--\ensuremath{\Delta}--X in the Brillouin zone, which supports the view of highly localized 3d electrons on the ${\mathrm{Mn}}^{2+}$ cations. The results of polarization-dependent and off-normal-emission measurements are also consistent with a localized Mn 3d orbital picture. To explain the presence of satellite photoemission peaks, a hybridization between Mn 3d and O 2p valence states must be invoked. The degree of hybridization is evident from resonant-photoemission measurements at the Mn 3p\ensuremath{\rightarrow}3d absorption threshold. The antiresonant behavior of the 3d-derived states near the top of the valence band in constant-initial-state spectra suggests that MnO is a charge transfer rather than a Mott insulator. The experimental results are discussed in the context of recent cluster and band theories which have been proposed to explain the insulating nature and electronic structure of MnO.

Imaging of 5f densities of states in resonant photoemission measurements.

Medium-resolution spectra (\ensuremath{\Delta}E=0.25 eV) at the 5f Fano resonance in uranium intermetallics are compared to spectra above and below the resonance region to show that the 5f (and 6d) spectral weight obtained from resonant photoemission (RP) compares well to the 5f spectral weight obtained at other photon energies. In well-hybridized systems, the 5f signal from RP gives an excellent representation of the 5f density of states (DOS). In narrow-band and localized systems, a satellite may appear in addition to 5f DOS-like structure, indicative of correlation effects.

Valence Band Electronic Structure of Cleaved Iron Oxide Single Crystals Studied by Resonant Photoemission

AbstractSynchrotron radiation has been used to perform resonant photoemission measurements across the 3p→3d photoabsorption threshold from cleaved FexO (x ≃0.945), Fe3O4, and α-Fe2O3 single crystal surfaces. The resonant enhancement of the Fe 3d photoelectrons allows the Fe 3d-derived final states in the valence band to be distinguished from the overlapping O 2p states. Using well-characterized single crystals, the distributions of Fe 3d-derived states associated with the ferrous (Fe2+) and ferric (Fe3+) cations have been identified. The Fe 3d-derived states are found to extend about 18 eV below the Fermi level in each oxide, which can be attributed to a significant amount of hybridization between the Fe 3d and O 2p orbitals.

5f-band width and resonant photoemission of uranium intermetallic compounds.

New experimental results and theoretical arguments are used in conjunction with previously published data to demonstrate that resonant photoemission (RPS) does not provide a reliable measure of the occupied $5f$ density of states in uranium intermetallic compounds. We implicate a resonant Auger process in this phenomenon and argue that RPS measurements (in conjunction with x-ray photoemission spectroscopy data) in this context are more useful as a qualitative guide to U $5f$-ligand hybridization.

Superconductivity, magnetism and valence fluctuations in rare earth-transition metal borides

Experiments on rare earth (R)-transition metal (T) borides of the type RRh 4B 4 and CeT 3B 2 are described. The interaction between superconductivity and competing types of magnetic order produce complex and interesting low temperature phase diagrams in pseudoternary RRh 4B 4 systems, formed by substituting another R element at the R sites, or a different T element at the Rh sites. The compounds CeT 3B 2, with T  Ru, Rh and Ir, display superconductivity, ferromagnetism and valence fluctuations, respectively. The low temperature phase diagram of the pseudoternary system formed between CeRu 3B 2 and CeRh 3B 2, and resonant photoemission measurements on these two compounds are discussed in connection with the anomalous ferromagnetism of CeRh 3B 2.

Electronic structure of CePd3 from resonant photoemission and optical reflectivity spectra

Optical reflectivity and resonant photoemission measurements on CePd3 yield a 4f binding energy Ef between 0.7 and 1.0 eV and set an upper limit of 0.3 eV on a surface shift of Ef. The 4f halfwidth at half maximum δ is between 0.5 and 0.8 eV. The spin fluctuation energy Γ = 20 meV is successfully related to Ef and δ by the Kondo impurity Ansatz.